It's not going to strengthen shins, but it will make the process of strengthening shins less agonizing
You strengthen the shins by kicking things with them. Start small - padded kali sticks, then take the pads off and whack your shins with 'em. Eventually you'll work up to PVC pipe or wood dummies. Some guys I know just go out and kick oak trees. They're kinda hardcore
Trick is to rub the jao on after you get done kicking things - it'll help keep the bruising down.
There are a few caveats here. There are different levels of jao, and there are different manufacturers of jao. Like anything that's unregulated by the government, you can make anything and call it jao. I caught one "asian herbalist" making it out of soy sauce and coffee. Smells right, but doesn't do jack. Ask around and find where in your area you can get the real stuff or, if you're ambitious enough, you can find recipes for it and make it yourself.
The jao will not enter your bloodstream from rubbing it on bruises - it can only enter your bloodstream if you put it in a cut, which you do NOT want to do.
Jao works differently for different people. For me it works great. My wife sees little effect.
As long as you don't drink it or get it in a cut, the only side effect is that you'll smell really strange for a day or so after using it. Oh, and it will stain just about anything it touches, so use with caution.